Untethered magnetic devices, such as magnetic microrobots and magnetically actuated capsule endoscopes, have become an active area of research because of their potential impact to minimally invasive medicine. These devices typically consist of a rigidly attached magnetic body on which magnetic forces and torques are applied by an external field. Some approaches to actuation utilize magnetic forces for pulling, while others apply torque generated by rotating magnetic fields to roll on a surface, swim through a fluid or crawl through a lumen via helical propulsion, or screw through soft tissue. Because these devices can be viewed as simple end-effectors of a larger robotic system, and they may range in size from the microscale to the mesoscale, such devices are referred to herein as magnetically actuated tools (MATs) without any implied size.
Currently, MATs are typically actuated using applied magnetic fields produced by electromagnets, often taking the form of orthogonal arrangements of Helmholtz or Maxwell coils. Such coils are generally small and work in a laboratory setting. However, scaling electromagnetic coils to sizes suitable to create fields for use with MATs inside a human body is currently difficult.
Because electromagnetic coil systems are expensive to scale to the size required for clinical use, researchers are considering actuation using a single permanent magnet outside of a patient's body. Although less expensive, actuation with permanent magnets is significantly more complex because applied magnetic force and torque cannot be controlled independently (although some force management strategies exist). In the case of MAT locomotion using rotating applied fields generated by a single rotating permanent magnet (RPM), the RPM is typically rotated around an axis {circumflex over (Ω)} such that the RPM's dipole moment M is always perpendicular to {circumflex over (Ω)} as depicted in FIG. 1.
As shown in FIGS. 1a-1b, when a dipole magnet with moment M rotates around the axis {circumflex over (Ω)} with M perpendicular to {circumflex over (Ω)}, the field vector at any given position rotates around, and is perpendicular to, a constant axis {circumflex over (ω)}. The axis ω at various positions are illustrated with arrows in (a). Any position on the {circumflex over (Ω)} axis is denoted to be in an axial position and any position in the plane spanned by the rotating M is a radial position. Representations of the field behavior at locations 1 and 2 are detailed in (b).
To date, MATs have been exclusively operated in radial or axial positions (relative to the RPM) where the applied field rotates around an axis parallel to {circumflex over (Ω)} (see FIG. 1a). The axial and radial positions have been historically favored for actuation due to their simplicity: the rotation axis {circumflex over (ω)} of the applied magnetic field in both positions lies parallel to the RPM axis of rotation, making it easy to visualize and characterize the coupling between the RPM and the MAT.
Requiring the MAT to be exclusively operated in these two positions, however, significantly constrains the physical placement of the RPM. This is because the large permanent magnet needs to follow the MAT when navigating through the human body and must be precisely positioned in order to achieve correct motion of the device. These workspace limitations become problematic and limit usefulness of RPMs in a clinical setting since the RPM must move during actuation to avoid collisions with the patient and other obstacles, and reposition for better control authority.